The traditional approach for cooling the rows of gas turbine stator vanes or nozzles has been to extract air from the intermediate stages of the gas turbine compressor. An external piping system is used to supply and control the air flow to the nozzles. In that arrangement, the cooling air circuit bypasses the turbine combustors where heat is supplied to the thermodynamic cycle. As a result, the diverted cooling air does not receive energy directly from the combustors and consequently does not completely expand through the turbine. This arrangement represents parasitic losses to the turbine output and degrades overall performance.
Steam cooling of the vanes of stator blades is not per se new and has been previously discussed, for example, in U.S. Pat. Nos. 4,314,442 and 4,565,490 to Rice. Steam cooling has also been discussed in a report by the Electric Power Research Institute, Report No. RP2620-1, entitled "Future Gas Turbine Development Options Definition Study," dated June, 1987. This report describes the anticipated performance improvement for steam cooling from a thermodynamic cycle analysis perspective. In the context of that report, the steam cooling supply requirements included a very high pressure source, i.e., on the order of 1840 psia, because it was then believed that such high pressure was needed to overcome circuit friction losses, as well as adverse rotational and centrifugal field forces associated with that proposed closed cooling circuit configuration. Nozzle cooling is also the subject matter of prior U.S. patent applications Ser. Nos. 07/794,032, title "Integrated Steam and Air for Combined Cycle Gas Turbines" and filed Nov. 19, 1991, by one of the co-inventors herein and also U.S. patent application Ser. No. 07/854,580, for "Integrated Steam and Air Cooling System for Gas Turbines," filed Mar. 20, 1992, also by the same co-inventor herein.
An alternate design configuration was reported by Matsuzaki et al., titled "New Advanced Cooling Technology and Material of the 1500 Class Gas Turbine," ASME 92-GT-240, 1992, which article describes a high-pressure steam cooling concept for the first-stage nozzle. The cooling concept disclosed in that article consisted of several peripheral cooling apertures in the air foil. The concept was similar to that originally described by Horner in "Development of High Temperature Turbine Subsystem Technology to a Technology Readiness Status Phase II," DOE/FE/1806 76, February, 1980. Both of these concepts, however, are very difficult to implement in complex highly curved air foil shapes. For example, the three-dimensional apertures extending along the periphery of the air foils are difficult to control from a manufacturing standpoint, thus leading to different cooling arrangements for the stator vanes in the gas turbine.
In a combined cycle operation, steam at several pressure and temperature levels is readily available. Coolant air in a gas turbine can be replaced by steam, which is a better cooling medium. Additionally, the problem of degradation of thermal efficiency associated with air as the cooling medium is minimized as air is replaced by steam. The inherently better properties of steam for cooling may also be used to increase the potential for higher firing temperatures in the gas turbine cycle.